Field of the Invention
The present invention relates to a hydrometallurgical Process for a nickel oxide ore, in more detail, in a hydrometallurgical Process for a nickel oxide ore using a High Pressure Acid Leach including a preparation step of ore slurry for making a slurry of a nickel oxide ore, a leaching step for obtaining leached slurry by transferring the relevant ore slurry to a High Pressure Acid Leach equipment for leaching nickel and cobalt, and a solid-liquid separation step of the relevant leached slurry, the present invention relates to a hydrometallurgical Process for a nickel oxide ore, which is capable of preventing inevitable operation shutdown of the above leaching step and maintaining high operation efficiency as a whole process basis, in a trouble of the steps other than the above leaching step.
Description of the Prior Art
A High Pressure Acid Leach using sulfuric acid has been noticed in recent years, as a wet-smelting process recovering Nickel and Cobalt from a nickel oxide ore, which contains an iron as a main component and 1 to 2% by weight of Nickel. This method is composed of consistent wet process steps without pyrometallurgical treatment steps such as drying and roasting steps, unlike the conventional general smelting method of nickel oxide ore, thus providing advantage in view of energy and cost saving. That is, the above High Pressure Acid Leach is capable of leaching nickel and cobalt selectively relative to iron, by controlling the redox potential and temperature of a leach solution inside a pressurized leaching reactor in the leaching step, and by fixing the iron, which is a main impurity, onto a leaching residue as a form of hematite (Fe2O3), therefore it has very big merit.
The above High Pressure Acid Leach is composed of a step for preparing an ore slurry, for example, by making slurry of a nickel oxide ore (ore slurry preparation step); a step for obtaining leached slurry, by adding the sulfuric acid to the above ore slurry, leaching at a high temperature of 220 to 280° C. under high pressure, using an autoclave (leaching step); a step for solid-liquid separation of a leaching residue and a leach solution containing nickel and cobalt (solid-liquid separation step); a step for neutralization separation of impurity elements such as iron, by adjusting the pH of the leach solution containing the impurity elements along with nickel and cobalt at 3 to 4 (a neutralization step); and a step for recovering of a mixed sulfide of nickel/cobalt, by supplying the hydrogen sulfide gas into the leach solution after neutralization separation (a sulfurization step) (for example, refer to Patent Literature 1).
Here, in the above ore slurry preparation step, the ore slurry, which is adjusted to predetermined particle size, is obtained by crushing and sieving of a nickel oxide ore with a wet process. In addition, in the above leaching step, firstly the ore slurry is subjected to passing through inside a multistage heater to increase temperature and pressure stepwise. Subsequently, in the autoclave, sulfuric acid is added to leach a part of the impurities, along with nickel and cobalt, into a solution under high pressure at high temperature formed with high-pressure steam. Lastly, the leach solution is subjected to passing through the multistage flash tank to decrease temperature and pressure stepwise.
It should be noted that, in the above leaching step, operation is performed at a high temperature of usually 240 to 260° C., under high pressure, by using a pressure vessel (autoclave) or the like, made of a very expensive material such as, for example, titanium, ceramic and stainless steel, therefore in the High Pressure Acid Leach equipment to be used in the operation, sufficient management based on standards is indispensable, for securing safety and maintenance of the equipment, in shutdown for scheduled inspection, repair or the like, or also start-up. For example, it is necessary to manage temperature increasing rate in start-up and temperature decreasing rate in shutdown at a nearly constant rate of equal to or lower than about 10 to 15° C./h, and not to exceed 25° C./h, even at the maximal rate.
For example, in start-up of the above High Pressure Acid Leach equipment, firstly, temperature of a heater, an autoclave and a flash tank, which compose the equipment, is increased in a state that each of them is separated individually. Still more, the inside of the autoclave is charged with water in advance, and temperature thereof is increased under an operation state of an attached apparatus such as a stirrer, seal thereof.
Next, at the timing when temperature of each apparatus is increased up to about 150 to 180° C., the heater, the autoclave and the flash tank are connected as apparatuses. It should be noted that, at this stage, because temperature is still low and suitable operation condition has not been attained, the ore slurry as a raw material cannot be supplied. Accordingly, industrial water is supplied to the heater instead of the ore slurry to be supplied in normal operation. Industrial water supplied here is sent from the flash tank to the subsequent preliminarily neutralizing step and the solid-liquid separation step, via the autoclave, and finally becomes discharged water.
After that, at the timing when temperature of each apparatus reached about 220° C., supply to the heater is switched from industrial water to the ore slurry. Still more, at the timing when the temperature reached about 230° C., sulfuric acid is added to the inside of the autoclave, to start leaching of nickel or cobalt from the ore slurry, and at this timing, start-up is completed, and hereafter normal operation is started.
It should be noted here that advantage of increasing temperature in a state that each of the heater, the autoclave and the flash tank is separated, is reduction of use amount of industrial water and amount of discharged water from the process. That is, in the case where they are connected at a temperature of below 150° C., for example, use amount of industrial water to be supplied instead of the ore slurry increases, resulting in generation of a large amount of discharged water. In this sense, it is desirable that they are connected at a temperature side as higher as possible near 180° C. However, in the case where they are connected at a temperature of over 180° C., for example, at equal to or higher than 200° C., difference between vapor pressure in the autoclave and vapor pressure in the flash tank becomes large, and thus pressure-like impact generates onto the flash tank in connection, and in the worst case, a valve or inner-lining brick of the flash tank are damaged.
On the other hand, on temperature decreasing in shutdown of the above High Pressure Acid Leach equipment, any of the methods may be selected from a method for cooling by supplying industrial water instead of the ore slurry, while these apparatuses are connected; or a method for cooling individually, by separation of each of the apparatuses.
In the former case, due to use of industrial water, discharged water generates in that amount, however, cooling rate is fast and thus temperature can be decreased in a shorter time. Here, temperature decreasing rate is generally 10 to 15° C./h. In addition, in the latter case, although there is no discharged water generation, cooling rate is slow and thus a longer time is required in temperature decrease. Here, temperature decreasing rate is generally 5 to 10° C./h. It should be noted here that ratio of both temperature decreasing rates is generally 3 times.
It should be noted here that the situation leading to inevitable shutdown of the above High Pressure Acid Leach equipment includes, other than shutdown accompanied with the above scheduled inspection and repair, the generation of emergency such as a process trouble, which could give tremendously bad influence on safety or environment; or the case of generation of a relatively small scale trouble such as off specifications of intermediate products in the ore slurry preparation step of a prior step of the leaching step, or the neutralization step or the sulfurization step and the like of the subsequent steps. In the case where such a trouble is generated, it was general that, in all of the cases other than the above emergency situation, the system is shutdown safely by gradually decreasing temperature and pressure, by taking importance on maintenance of the equipment, similarly as in shutdown accompanied with the above scheduled inspection, repair or the like.
However, in a trouble of the steps other than the above leaching step using the High Pressure Acid Leach equipment, the above relatively small scale troubles generate in many cases, and in such troubles, because of no failure in the High Pressure Acid Leach equipment itself, there was required that, by maintaining operation of the High Pressure Acid Leach equipment as longer as possible, operation shutdown time shall be shortened, and decrease in operation efficiency shall be suppressed, as well as frequency of shutdown and start-up of the equipment shall be decreased. As a countermeasure for this, there has been performed a method for maintaining operation, by using buffer tanks installed before and after the High Pressure Acid Leach equipment, to temporarily store a solution inside the buffer tanks, on the contrary, a method for maintaining operation by using the solution inside the buffer tanks, or a method for preventing shutdown of the High Pressure Acid Leach equipment, by adjustment of flow rate of slurry to be supplied into the autoclave. However, even by such countermeasures, shutdown of the leaching step was necessary, in the case of over limitation.
Under the above circumstance, it has been required, a trouble of the steps other than the above leaching step using the High Pressure Acid Leach equipment, to prevent operation shutdown of the leaching step, which is configured by the High Pressure Acid Leach equipment, and to maintain high operation efficiency as a whole process basis.    [Patent Literature 1] JP-A-2005-350766 (page 1 and page 2)